Transcript Document

TC Lifecycle and Intensity Changes
Part II: Intensification
Hurricane Katrina (2005)
August 24-29
Tropical
M. D. Eastin
Outline
Tropical Cyclone Intensification
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Tropical
Large-Scale Factors
Symmetric Route
Asymmetric Route
Maximum Potential Intensity (MPI)
Eyewall Replacement Cycles
Role of Trough Interactions
Role of Upper Ocean Features
Rapid Intensification
M. D. Eastin
TC Intensification
Intensity change can be a slow and steady process or it can occur
rapidly over the course of several hours
Forcing exists on multiple scales
• Seasonal (SST, relative humidity)
• Synoptic (wind shear)
• Mesoscale (convective features, MCV, eyewall cycles)
• Microscales (air-sea interface, water phase changes)
Complex interactions exist between the scales
Very difficult forecast problem!!!
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TC Intensification: Large Scale Factors
Conditions favorable for intensification:
• Low vertical wind shear pattern
• Moist mid-troposphere
• Warm ocean with a deep mixed layer
• Enhanced outflow
• Persistent deep convection near the cyclone center
Conditions favorable for weakening would be the opposite
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Symmetric Route to Intensification
Local Heat and Momentum Sources:
• In 1982, Lloyd Shapiro and Hugh Willoughby examined
the response of “balanced” (slowly evolving), symmetric
hurricanes to local sources of heat and momentum
• Idealized study
(built upon many before)
Hugh Willoughby
• Symmetric vortex is in thermal wind balance
• The eyewall is a uniform ring of convection
• Local heat sources (mimic latent heat release in
convection)
• Local momentum sources (mimic vertical advection
of momentum to upper levels by convection)

In hurricane-like vortices, the local sources induce
secondary circulations that can slowly intensify the
vortex ...How?
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No Picture
Available
Lloyd Shapiro
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Symmetric Route to Intensification
Local Heat Sources:
• Heating produces a local temperature
anomaly (like a buoyant updraft) which
disturbs the local pressure surfaces
Streamfunction response
to a local heat source
(mathematical solution)
• This effect on the local pressure surfaces
induces an local secondary circulation
• In hurricanes, the inner circulation is more
confined with radius than the outer
Streamfunction response to a
local heat source in the mid-level eyewall
(numerical simulation)
H
Adiabatic
Warming
Adiabatic
Warming
L
Note the difference between the two circulations
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Symmetric Route to Intensification
Local Heat Sources:
• The sinking branches adiabatically
warm the air (further pressure
decreases)
• The radial confinement of the inner
circulation limits the warming to a
smaller area than that associated
with the outer circulation
Streamfunction response
Change in pressure and tangential wind by
local heat source in the mid-level eyewall
(numerical simulation)
Lowers pressure in the eye
Increases winds in the eyewall
Radius of the local heat source is denoted
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M. D. Eastin
Symmetric Route to Intensification
Local Momentum Sources:
• Increased tangential momentum results
in a “super-gradient” state and an outward
acceleration up the pressure gradient
Streamfunction response
to a local momentum source
(mathematical solution)
• This acceleration produces an local
secondary circulation to conserve mass
PGF
Gradient
Balance
Tropical
PGF
L
Streamfunction response to a local
momentum source in the upper-level eyewall
(numerical simulation)
Centrifugal
Force
L
Super
Gradient
State
H
H
Centrifugal
Force
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Symmetric Route to Intensification
Local Momentum Sources:
• The lower circulation’s inflow
conserves angular momentum
(increases the tangential wind)
• The upper circulation’s descent
results in adiabatic warming
confined in the eye
(lowers pressure)
Streamfunction response
Change in pressure and tangential wind by
local momentum source in the upper-level eyewall
(numerical simulation)
Lowers pressure in the eye
Increases winds in the eyewall
Radius of local momentum source is denoted
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M. D. Eastin
Asymmetric Route to Intensification
Convective Bursts:
• In 1960, Joanne Malkus (Simpson) and Herbert Riehl
first suggested that hurricane evolution was linked
to a few, asymmetric, intense cumulonimbus clouds,
which they called “hot towers”, that carried a large
fraction of the high-θe inflow aloft in undiluted updrafts
• Observational study
Joanne Simpson
• Eyewall convection was often asymmetric with
many localized updraft cores
• Convection was often episodic with “bursts”
 These “convective bursts” increase the latent heating
aloft and the asymmetric secondary circulations
that can intensify the vortex...How?
Herbert Riehl
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Asymmetric Route to Intensification
Convective Burst in Hurricane Bonnie (1998) on 23 August
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Asymmetric Route to Intensification
Convective Bursts:
Conceptual Model of Convective Burst
• Overshooting and diverging convection at
upper levels drives asymmetric mesoscale
descent (adiabatic warming) in the eye,
which lowers the pressure, increasing the
pressure gradient and tangential winds
• A recent survey of convective bursts:
• 80% of TCs have at least one “burst”
• 70% of TCs intensify after a “burst”
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Maximum Potential Intensity
Maximum Potential Intensity (MPI)
Emanuel (1988)
• MPI is primarily a function of SST and
the mean outflow temperature at the
top of the eyewall
• No eye subsidence
960
940
Minimum Pressure (mb)
• Theoretical maximum intensity a TC could
achieve if environmental conditions were
infinitely perfect
MPI computed for Typical Conditions
920
900
880
860
Observed
840
Emanuel
820
Holland
800
26.0
Holland (1998)
• MPI is primarily a function of
environmental CAPE
• Incorporates eye subsidence for
strong hurricanes
Tropical
26.5
27.0
27.5
28.0
28.5
29.0
29.5
30.0
SST (C)
Note: Observed values should be higher
since the dynamical environment
will limit TC intensities
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Eyewall Replacement Cycles
Eyewall Replacement Cycles:
• Outer eyewall develops and begins to contract
• Inner eyewall begins to dissipate
• Maximum winds decrease
• Minimum central pressure increases
• Outer eyewall continues to contract
• Maximum winds increase
• Minimum central pressure decreases
Hurricane
Gilbert (1988)
Radar at 2300 UTC
13 September
Tangential Winds
11-16 September
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Eyewall Replacement Cycles
Eyewall Replacement Cycles: Statistics
• More common in intense tropical cyclones
• Process typically takes 36 hours
• Survey of multiple eyewall structures in
TCs with maximum winds > 120 knots
(Category 345) during 1997-2002
• 40% of Atlantic hurricanes
• 60% of East Pacific hurricanes
• 70% of West Pacific typhoons
• Significant factor in TC intensity changes
• Results in an outward expansion of the
wind field (i.e., TC grows in size) and an
“annular” (or symmetric) wind field
• An eyewall replacement cycle contributed
the weakening of Katrina (2005) just prior
to landfall near New Orleans
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Eyewall Replacement Cycles
Eyewall Replacement Cycles: Hurricane Ivan (2004)
Note the overall
expansion
of the wind field
after 6 EWRCs
Inner eyewall
Secondary eyewall
Third eyewall
From Sitkowski
et al. (2011)
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Role of Trough Interactions
Basic Idea:
• Upper tropospheric troughs can promote
intensification by enhancing the
upper-level divergence and outflow
• Troughs can also promote weakening by
enhancing the vertical shear
experienced by the TC
Vorticity Cross-Section
Upper-level Trough
• What are the differences between “good”
and “bad” troughs (for intensification)?
Hurricane
Dennis
(1999)
Hanley et al. (2001):
• Examined 146 TCs which interacted with upper-level troughs
• 68% of the TCs intensified
• Composited the large-scale flow with respect to each TC center
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Role of Trough Interactions
Favorable Trough Interactions:
• Trough potential vorticity (PV)
maximum comes within 400 km
of TC center, but rarely closer
Composite 200 mb Flow
and Potential Vorticity
• Troughs are generally small in size
• Outflow is enhanced
• Mean vertical wind shear between
850 and 200 mb is less than 8 m/s
Note: Asterick denotes TC center
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Role of Trough Interactions
Unfavorable Trough Interactions:
• Trough potential vorticity (PV)
maximum comes within 100 km
of TC center
Composite 200 mb Flow
and Potential Vorticity
• Troughs are generally larger in size
• Mean vertical wind shear between
850 and 200 mb is greater than 10 m/s
Note: Asterick denotes TC center
Tropical
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Role of Upper Ocean Features
Deep Warm Currents and Eddies:
SST on 8-25-05
• A shallow oceanic mixed layer can easily
be eroded by TC induced upwelling of
cold water, resulting in cold SSTs and
and the potential weakening of the TC
• A deep oceanic mixed layer will experience
less upwelling of cold water, resulting in
higher SSTs, and a better chance for
intensification
Depth of 26ºC on 8-25-05
Deep warm water matters, not just SST
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Role of Upper Ocean Features
Common Deep Warm Currents and Eddies:
Gulf Stream
Warm
Core
Eddies
(Rings)
Loop
Current
Trajectories of
NOAA buoys
from
1978-2003
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Rapid Intensification (RI)
Definition and Statistics:
• Increase in maximum wind speed of 15.4 m/s (30 knots) over a 24 hour period
• A survey of Atlantic basin TCs (1989-2000)
• All category 4 and 5 hurricanes underwent a period of RI during their life
• ~60% of all hurricanes undergo a period of RI
• ~30% of all tropical storms undergo RI
When is Rapid Intensification more likely?
• Storm is far from it’s MPI (weak system)
• Storm is small in size (wind max or eye < 75 km)
• Storm over high SST and deep warm oceanic mixed layer
• Higher than normal mid-tropospheric humidity
• Low vertical wind shear
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Rapid Intensification (RI)
Hurricane Opal (1995)
•
Weak hurricane stalled in
southern Gulf of Mexico
•
Moved rapidly NE during the
night of 4 October
•
Rapidly intensified from 965
to 916 mb in 14 hours
•
Coastal residents not warned
appropriately (unexpected
intensification)
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Rapid Intensification (RI)
Hurricane Opal (1995)
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Rapid Intensification (RI)
Forecasting: 37-GHz Imagery
• Kieper and Jiang (2012) evaluated
precipitation patterns prior to and
during RI for 84 Atlantic TCs
Ring of shallow precipitation
around a small “eye”
 Rapid intensification often occurred
6-12 hrs after the first appearance
of a “ring pattern” in the 37-GHz
passive microwave (SSMI) imagery
(75% of all RI cases in 2003-2007)
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TC Lifecycle and Intensity Changes
Part II: Intensification
Summary
• Large-Scale Factors
• Symmetric Intensification (assumptions, physical processes, cases)
• Intensification via Hot Towers (assumptions, physical processes)
• MPI (basic idea)
• Eyewall Replacement Cycles (process, impacts)
• Upper-level Trough Interactions (favorable/unfavorable, impacts)
• Upper Ocean Features (examples, physical processes, impacts)
• Rapid Intensification (definition, favorable situations, forecasting)
Tropical
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References
Bosart, L. A., C. S. Velden, W. E. Bracken, J. Molinari, and P. G. Black, 2000: Environmental influences on the rapid
intensification of Hurricane Opal (1995) over the Gulf of Mexico. Mon. Wea. Rev., 128, 322-352
Carrasco, C. A., C. W. Landsea, and Y.-L. Lin, 2014: The influence of tropical cyclone size on it is intensification.
Weather and Forecasting, 29, 582-590.
Emanuel, K. A., 1988: The maximum intensity of hurricanes. J. Atmos. Sci., 45, 1143-1155.
Hanley, D. E., J. Molinari, and D. Keyser, 2001: A composite study of of the interactions between tropical cyclones and
upper-tropospheric troughs. Mon. Wea. Rev., 129, 2570-2584.
Heymsfield, G. M., J. B. Halverson, J. Simpson, L. Tian, and T. P. Bui, 2001: ER-2 Doppler radar investigations of the
eyewall of Hurricane Bonnie during the Convection and Moisture Experiment-3. J. Appl. Met., 40, 1310-1330.
Holland, G. J., 1997: The maximum potential intensity of tropical cyclones. J. Atmos. Sci., 54, 2519-2541.
Kaplan, J., and M. DeMaria, 2003: Large-scale characteristics of rapidly intensifying tropical cyclones in the north Atlantic
basin. Wea. Forecasting, 18, 1093-1108.
Kieper, M., and H. Jiang, 2012: Predicting tropical cyclone rapid intensification using the 37-GHz ring pattern identified from
passive microwave measurements, Geophysical Research Letters, 39, L13804.
Kossin, J. P., and M. D. Eastin, 2001: Two distinct regimes in the kinematic and thermodynamic structure of the hurricane
eye and eyewall. J. Atmos. Sci., 58, 1079-1090.
Kossin, J. P., and M. Sitkowski, 2012: Predicting hurricane intensity and structure changes associated with eyewall
replacement cycles, Wea. Forecasting, 27, 484-488.
Knaff, J. A., M. DeMaria, and J. P. Kossin, 2003: Annular hurricanes. Wea. Forecasting, 18, 204–223.
Malkus, J., and H. Riehl, 1960: On the dynamics and energy transformations in steady-state hurricanes. Tellus, 12, 1–20.
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References
Moeller, D. J., and M. T. Montgomery, 1999: Vortex Rossby Waves and hurricane intensification in a barotropic model.
J. Atmos. Sci., 56, 1674-1687.
Montgomery, M. T., and R. J. Kallenbach, 1997: A theory for vortex Rossby waves and its application to spiral bands and
intensity changes in hurricanes. Quart. J. Roy. Meteor. Soc., 123, 435–465.
Shapiro, L. J., and H. E. Willoughby, 1982: The response of balanced hurricanes to local sources of heat and momentum.
J. Atmos. Sci., 39, 378–394.
Sitkowski, M. J. P. Kossin, and C. M. Rozoff, 2011: Intensity nad structure changes during eyewall replacement cycles.
Mon. Wea. Rev., 139, 3829-3847.
Sitkowski, M. J. P. Kossin, and C. M. Rozoff, 2011: Intensity nad structure changes during eyewall replacement cycles.
Mon. Wea. Rev., 139, 3829-3847.
Willoughby, H. E., and M. L. Black, 1992: The concentric eyewall cycle of Hurricane Gilbert. Mon. Wea. Rev., 120, 947-957
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